<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        #div1 {
            width: 300px;
            height: 300px;
            background-color: #ccc;
            /*一定要绝对定位*/
            position: absolute;
        }
    </style>
</head>

<body>
    <div id="div1"></div>
</body>

</html>
<script>
    window.onload = function () {
            var div1 = document.getElementById("div1");
            div1.onmousedown = function (ev) {
                var oevent = ev || event;

                var distanceX = oevent.clientX - div1.offsetLeft;
                var distanceY = oevent.clientY - div1.offsetTop;

                document.onmousemove = function (ev) {
                    var oevent = ev || event;
                    div1.style.left = oevent.clientX - distanceX + 'px';
                    div1.style.top = oevent.clientY - distanceY + 'px';
                };
                document.onmouseup = function () {
                    document.onmousemove = null;
                    document.onmouseup = null;
                };
            };
        }
</script>